Ouch! You know the feeling.
Stub your toe or bite into your tongue, and the sensation of pain
shoots through your body. But what exactly is pain? To understand,
we'll have to look just under the surface of your skin. Don't worry
... it won't hurt.

You have four distinctly
different types of receptors beneath your skin - for pain, pressure,
heat, and cold. (You also have chemical receptors in your tongue
and inside your nose, and photoreceptors in the eye. Some animals
can also sense electrical and magnetic fields.) Each receptor has
dendrites that receive input and change it to an electrical message.

Here's how they work.
Imagine you prick your skin with a pin. Cells are damaged. They
release chemicals into their surroundings, and those chemicals cause
changes in the membranes of dendrites. Within your brain, the injury
message is sorted out from other messages and interpreted as pain.

What happens if you rub
your injured skin, or warm it? These messages stimulate other receptors,
but their signals travel the same pathway as the pain signals, masking
them. Doctors use this technique to help those who suffer from chronic
pain. They implant a device that sends a constant tingle along the
path to the brain, muffling the pain signal for the patient.